As the advantages of fiber optic based communication and control of industrial processes becomes better known, increasing emphasis is being placed on various methods of simple, inexpensive, and reliable communication of optically sensed physical parameters, or measurands. Optical analysis of certain fluid materials offers known improvements over other techniques.
The measurement of the light transmitting or light scattering properties of a fluid ordinarily requires that a beam of light or radiant energy be passed through the fluid and subsequently directed towards a radiant energy detector. Optical apparatus for accomplishing this task have been used in which discrete components such as lenses, mirrors, or internally reflecting light guides are employed for the sampling apparatus. Optical fibers may be used to convey the light to the sensing apparatus and back to detection equipment. Examples of such techniques are illustrated in U.S. Pat. Nos. 4,591,268 to Lew ('268); U.S. Pat. No. 4,320,978 to Sato ('978); and U.S. Pat. No. 4,152,070 to Kushner et al ('070). These methods are generally unsuited for direct submersion within the test fluid because the optical surfaces are derogated by fluid contact, i.e., dirt erosion, pitting, and dissolving of the surfaces.
The use of fiber optic light guides is recognized for permitting the measurement of the light transmitting or scattering properties of fluids in harsh environments, such as a process container or pipeline containing the fluid of interest. Thus, U.S Pat. No. 4,040,743 to Villaume et al ('743) and U.S. Pat. No. 4,561,779 to Nagamune et al ('779) depict apparatus for the in-situ measurement of fluid suspensions. A similar approach described by H. Raab in Technisches Messen, 50, 1983(12), p. 475, is employed for the in-situ assay of certain fluids. A common feature of these known methods is the use of relatively small prisms having planar surfaces which act to bend a light beam through 90 degrees. Such prisms can be expensive to fabricate and difficult to align.
Conical reflecting elements have been previously described in the literature (cf. M. Rioux, et al, Applied Optics, 17(10), 1978, p. 1532). Their use has been primarily as imaging devices for objects disposed along the conical reflecting element's axis of revolution. As will become evident from the subsequent disclosure, the method and apparatus of the invention described herein depart from these known configurations and permit utilization of the interior conical reflecting surface in an off-axis manner.
In addition, since the present invention has application in the fermentation arts, it is useful and often necessary to minimize bubbles in the measurement area. Known passive bubble reducing techniques are inadequate when applied to a fermentor environment. Typically intricate and narrow passageways designed to promote drainage of foamy samples are ineffective, and may be prone to blockage from the solution, which is typically cell-laden.
For this reason, the present invention comprehends the inclusion of a valved still well or stilling chamber from which the bubbles and foam are effectively drained prior to measurement. The combination probe thus incorporates a stilling well chamber, which may be either electrically or pneumatically valved, and a novel optical probe. Such a valved still well embodiment includes an `open` position in which the solution is free to pass through the measurement chamber, and a `closed` position in which the bubbles and/or foam in the solution are permitted to drain briefly before the measurement.
The prior art method of measuring free fatty acid content in oils typically requires a skilled technician to perform a bench titration on a process sample according to American Oil Chemists' Society Official (A.O.C.S.) Method Ca 5a-40. While this method is routine, determination of the endpoint on which the test depends is somewhat subjective. Since free fatty acid content is one of the most important factors affecting the quality of, for example, edible oils and fats, it is important that the tests be quickly, reliably, and consistently performed. Previous on-line methods suffer from various analysis faults, high operator skill levels, and/or costly and frequent maintenance schedules.
On-line titration is discussed in "Is On-Line Titration the Answer?", INTECH, Feb. 1989, pp. 39-41 and in a paper "FFA Determination Using an On-Line Titrator and a Colorimetric Sensor", by C. Cheney et al, given May 23, 1989 at Sensor Expo West, May 1989. Opto-electronic titration is discussed in "Construction and Performance of a Fluorimetric Acid-Base Titrator with a Blue LED as a Light Source", by O. S. Wolfbeis et al, ANALYST, Nov. 1986, pp. 1331-1334.
A sequence and method of operation of an on-line titrator suitable for performing FFa analysis is discussed in "FFA Determination Using an On-Line Titrator and a Colorimetric Sensor". The usual method of endpoint detection for automated acid-basetitrations is a pH measurement. This is unsuitable for the FFa analysis because the non-aqueous titration medium dehydrates the electrode within a few days. Furthermore, the output of the pH measurement apparatus (i.e., titration curve) does not remain constant as the titration proceeds towards completion, but rather increases slowly throughout the titration with the occurrence of the endpoint being indicated by an increase in the rate of change of the pH measurement. Such a titration curve is difficult to interpret with simple electronic discriminator circuitry such as used in the titrator/analyzer used herein to detect the occurrence of the endpoint.
For the purposes of this limited description, "fiber optic", "optical fiber", "light guide", and "radiant energy pathway" refer to optical communication paths, generally optical fibers. As used herein, the terms "radiant energy" and "light" are used interchangeably to refer to electromagnetic radiation of wavelengths between 3.times.10.sup.-7 and 10.sup.-9 meters, and specifically includes infrared, visible, and ultraviolet light. For simplicity, such electromagnetic radiation may be referred to as simply "light." These terms specifically include both coherent and non-coherent optical power. "Monochromatic" refers to radiant energy composed substantially of a single wavelength. "Collimated" light refers to radiant power having rays which are rendered substantially parallel to a certain line or direction. "FFA" refers to free fatty acid, the product proportion measurement of which is the subject of the disclosed specific application of the combination probe/ analyzer.